Understanding the Layers

Layer 1: Physical

(Processing)

Devices: Cables, hubs

Protocols: Ethernet, cabling portocols

The ________ layer is associated with the physical hardware. It includes specifications for cable types, such as 1000BaseT, connectors, and hubs. Computing devices such as computers, servers, routers, and switches transmit data onto the transmission medium in a bitstream. This bitstream is formatted according to specifications at higher-level OSI layers.

Layer 2: Data Link

(Data)

Devices: Switches

Protocols: MAC, ARP, VLANs

The ____ ____ layer is responsible for ensuring that data is transmitted to specific devices on the network. It formats the data into frames and adds a header that includes media access control (MAC) addresses for the source and destination devices. It adds frame check sequence data to the frame to detect errors, but it doesn’t support error correction. The ____ ____ layer simply discards frames with detected errors. Flow control functions are also available on this layer.

Traditional switches (Layer 2 switches) operate on this layer. Computer network interface cards have a MAC assigned, and switches map the computer MAC addresses to physical ports on the switch. Systems use the Address Resolution Protocol (ARP) to resolve IPv4 addresses to MAC addresses. VLANs are defined on this layer.

Layer 2 attacks as Address Resolution Protocol (ARP) poisoning, media access control (MAC) flooding, and MAC cloning.

Layer 3: Network

(Need)

Devices: Router, Layers 3 switch

Protocols: IPv4, IPv6, IPsec, ICMP

The _______ layer uses logical addressing in the form of IP addresses at this layer. This includes both IPv4 addresses and IPv6 addresses. Packets identify where the traffic originated (the source IP address) and where it is going (the destination IP address). Other protocols that operate on this layer are IPsec and ICMP. Routers and Layer 3 switches operate on this layer.

Layer 4: Transport

(To)

Protocols: TCP, UDP

The _________ layer is responsible for _________ing data between systems, commonly referred to as end-to-end connections. Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) operate on this layer. TCP provides reliability with error control, flow control, and segmentation of data.

Layer 5: Session

(Seem)

The _______ layer is responsible for establishing, maintaining, and terminating sessions between systems. In this context, a _______ refers to an extended connection between two systems, sometimes referred to as dialogues or conversations. As an example, if you log on to a webpage, the _______ layer establishes a connection with the web server and keeps it open while you’re interacting with the webpages. When you close the pages, the _______ layer terminates the _______.

If you’re like many users, you probably have more than one application open at a time. For example, in addition to having a web browser open, you might have an email application open. Each of these is a different _______, and the _______ layer manages them separately.

Layer 6: Presentation

(People)

The ____________ layer is responsible for formatting the data needed by the end-user applications. For example, American Standard Code for Information Interchange (ASCII) and Extended Binary Coded Decimal Interchange Code (EBCDIC) are two standards that define codes used to display characters on this layer.

Layer 7: Application

(All)

Devices: Proxy servers, web application firewalls, next-generation firewalls, UTM (unified threat management) security appliances, and web security gateways

Protocols: DNS, FTP, FTPS, SFTP, TFTP, HTTP, HTTPS, IMAP4, LDAP, POP3, SFTP, SMTP, SNMP, SSH, and TFTP

The __________ layer is responsible for displaying information to the end user in a readable format. ___________ layer protocols typically use this layer to determine if sufficient network resources are available for an __________ to operate on the network.

Note that this layer doesn’t refer to end-user ___________ directly. However, many end-user applications use protocols defined at this layer. For example, a web browser interacts with DNS services to identify the IP address of a website name. Similarly, Hypertext Transfer Protocol (HTTP) and HTTP Secure (HTTPS) transmit webpages over the Internet on this layer.

Some of the protocols that operate on this layer are:

• HTTP and HTTPS

• Secure Shell (SSH)

• Domain Name System (DNS)

• Post Office Protocol 3 (POP3)

• Simple Mail Transfer Protocol (SMTP)

• File Transfer Protocol (FTP) and FTP Secure (FTPS)

• Secure FTP (SFTP) and Trivial FTP (TFTP)

• Internet Message Access Protocol 4 (IMAP4)

• Simple Network Management Protocol (SNMP)

• Lightweight Directory Access Protocol (LDAP) and LDAP Secure (LDAPS)

Many advanced devices are ___________-aware and operate on all of the layers up to the ___________ layer. This includes proxy servers, web ___________ firewalls, next-generation firewalls (NGFWs), unified threat management (UTM) security appliances, and web security gateways.